ALBERT

Description: The Galactic Centre hosts a puzzling stellar population in its inner few parsecs, with a high abundance of surprisingly young, relatively massive stars bound within the deep potential well of the central supermassive black hole, Sagittarius A* (ref. 1). Previous studies suggest that the population of objects emitting soft X-rays (less than 10 kiloelectronvolts) within the surrounding hundreds of parsecs, as well as the population responsible for unresolved X-ray emission extending along the Galactic plane, is dominated by accreting white dwarf systems. Observations of diffuse hard-X-ray (more than 10 kiloelectronvolts) emission in the inner 10 parsecs, however, have been hampered by the limited spatial resolution of previous instruments. Here we report the presence of a distinct hard-X-ray component within the central 4 x 8 parsecs, as revealed by subarcminute-resolution images in the 20-40 kiloelectronvolt range. This emission is more sharply peaked towards the Galactic Centre than is the surface brightness of the soft-X-ray population. This could indicate a significantly more massive population of accreting white dwarfs, large populations of low-mass X-ray binaries or millisecond pulsars, or particle outflows interacting with the surrounding radiation field, dense molecular material or magnetic fields. However, all these interpretations pose significant challenges to our understanding of stellar evolution, binary formation, and cosmic-ray production in the Galactic Centre.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Perez, Kerstin -- Hailey, Charles J -- Bauer, Franz E -- Krivonos, Roman A -- Mori, Kaya -- Baganoff, Frederick K -- Barriere, Nicolas M -- Boggs, Steven E -- Christensen, Finn E -- Craig, William W -- Grefenstette, Brian W -- Grindlay, Jonathan E -- Harrison, Fiona A -- Hong, Jaesub -- Madsen, Kristin K -- Nynka, Melania -- Stern, Daniel -- Tomsick, John A -- Wik, Daniel R -- Zhang, Shuo -- Zhang, William W -- Zoglauer, Andreas -- England -- Nature. 2015 Apr 30;520(7549):646-9. doi: 10.1038/nature14353.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉1] Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, Room 1027, New York, New York 10027, USA [2] Haverford College, 370 Lancaster Avenue, KINSC L109, Haverford, Pennsylvania 19041, USA. ; Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, Room 1027, New York, New York 10027, USA. ; 1] Instituto de Astrofisica, Facultad de Fisica, Pontificia Universidad Catolica de Chile, 306, Santiago 22, Chile [2] Millennium Institute of Astrophysics, Vicuna Mackenna 4860, 7820436 Macul, Santiago, Chile [3] Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA. ; Space Science Laboratory, UC Berkeley, 7 Gauss Way, Berkeley, California 94720, USA. ; Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 37-555, Cambridge, Massachusetts 02139, USA. ; DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark. ; 1] Space Science Laboratory, UC Berkeley, 7 Gauss Way, Berkeley, California 94720, USA [2] Lawrence Livermore National Laboratory, PO Box 808, Livermore, California 94551-0808, USA. ; Cahill Center for Astronomy and Astrophysics, 1200 East California Boulevard, MC 290-17, California Institute of Technology, Pasadena, California 91125, USA. ; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-83, Cambridge, Massachusetts 02138, USA. ; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, 4800 Oak Grove Drive, MS 169-221, California 91109, USA. ; NASA Goddard Space Flight Center, 8800 Greenbelt Road, Code 662, Greenbelt, Maryland 20771, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25925477" target="_blank"〉PubMed〈/a〉

Description: We present a new way of describing the flares from Sgr A* with a self-consistent calculation of the particle distribution. All relevant radiative processes are taken into account in the evolution of the electron distribution and resulting spectrum. We present spectral modelling for new X-ray flares observed by NuSTAR , together with older observations in different wavelengths, and discuss the changes in plasma parameters to produce a flare. We show that under certain conditions, the real particle distribution can differ significantly from standard distributions assumed in most studies. We conclude that the flares are likely generated by magnetized plasma consistent with our understanding of the accretion flow. Including non-thermal acceleration, injection, escape, and cooling losses produces a spectrum with a break between the infrared and the X-ray, allowing a better simultaneous description of the different wavelengths. We favour the non-thermal synchrotron interpretation, assuming the infrared flare spectrum used is representative. We also consider the effects on Sgr A*'s quiescent spectrum in the case of a density increase due to the G2 encounter with Sgr A*.